There are mostely 2 reasons why your activity could not be uploaded on Strava:
1) Activity without any GPS route: If your activity has been recorded without a GPS route it will be rejected when on Strava. This can happen in 2 cases:
The ActiveLook application misses authorisations to access location information and is not able to generate a GPS route. To fix this, please refer to one of the 2 articles:
The Activity is so short that no GPS point could be logged. You may delete this session.
2) Missing / bad internet connection : Your mobile phone needs an internet connection for the upload to happen. Please ensure mobile network or wifi is available and retry the upload.